This invention relates to sensing loads in a mechanical interface between a surgical robot arm and a surgical instrument.
In designing a robot arm for performing surgical procedures, one desirable characteristic is for the arm to be relatively small and light-weight. These characteristics provide a number of advantages: for example that the arm is easier for a technician to position before surgery takes place, that more arms can be fitted closely together around a surgical site, and that the forces required to move the arms are less than with bulkier devices. It is particularly desirable to reduce size and weight at the distal end of the arm. Since instruments are normally attached at the distal end of the arm, that implies reducing the size and weight of the arm-to-instrument interface.
Instruments for use in robotic surgery may have various mechanical elements which can be moved under the control of the robot. The interface provided on the arm may include on its exterior one or more movable mechanical elements which can couple to corresponding elements on the instrument when the instrument is in place on the interface. Those movable elements on the arm can be driven by motors or other actuators in the arm, and that motion can be transferred through the interface to the corresponding elements on the instrument. In that way the mechanical elements on the instrument can be driven from the arm. It is desirable for the control system of the robot to be able to receive feedback on the position of the mechanical drive to the instrument and on the force being applied through that drive. One way to do this is to provide one or more force sensors on the arm-to-instrument interface.
A difficulty with providing such sensors is that first they should be small and light, so as to avoid making the arm more bulky; but also, since the forces applied in surgery can be relatively small, they should be relatively sensitive and accurate. This requires careful design of the sensors. In addition, when multiple sensors are provided to sense force on multiple force paths it is desirable for the arrangement to avoid force on one path influencing the measurement on another path.
FIG. 1 shows one possible arrangement for sensing force in an instrument drive of a surgical arm. A motor 1 is attached to a lead screw 2, so that the motor can rotate the lead screw about its axis. The shaft of the lead screw comprises a threaded portion 3 and a non-threaded portion 4. Bearings 5, 6 are bonded to the non-threaded portion in such a way that they cannot slide along the shaft. A follower nut 8 is threaded onto the lead screw. The follower nut is restrained, for example by running in a slot defined in an exterior wall of the arm, so that it cannot rotate when the lead screw is turned by the motor. As a result, turning the lead screw 2 by means of the motor 1 results in the follower nut 8 translating along the axis of the lead screw. The follower nut has a formation 9 which can mate with a corresponding formation on an instrument in order to drive motion of a part of the instrument when the instrument is fixed to the surgical arm.
A load cell 7 is located between the bearings 5, 6. The load cell is shown in more detail in FIG. 2. The load cell comprises an annular outer housing 10. A membrane 11 is suspended across the interior of the housing. Strain gauges 13, 14 are fixed to the membrane so as to sense distortion of the membrane. The strain gauges provide an electrical output indicative of the strain on the membrane. At the centre of the membrane is a hole 12. Load cells for this type are commercially available, for example the Emsyst EMS 70. Other types of load cell include the FOWA-1 annular load cell from Müller Industrie-Elektronik GmbH. These load cells are used for applications like measuring cable tension or bolt compression.
When the load cell is attached to the instrument drive of FIG. 1 the membrane is held between the bearings 5, 6, with the shaft of the lead screw passing through hole 12. The housing 10 is attached to the body of the robot arm. In this way, the lead screw is at least partially supported from the body of the robot arm by the load cell 7. When a force is applied from an instrument along the axis of the lead screw, that force is transmitted through formation 9 to the follower nut 8. The pitch of the thread of the lead screw is such that the axial force does not cause the lead screw to rotate. Instead the force is transmitted by the bearings 5, 6, which are longitudinally fast with the lead screw, to the membrane of the load cell. That force can then be detected by the strain gauges 13, 14. This provides an indication of the force applied by the instrument.
In order to drive all the motions of a more complex instrument the robot arm can have a number of such instrument drives. As shown in FIG. 1, the load cell can be the most radially prominent part of the instrument drive. Improving the packaging of the load cell can therefore contribute to the compactness of the arm-to-instrument interface. When the interface has a number of drives of the type shown in FIG. 1, one way to package them efficiently is to stagger the drives so that the load cell of each drive is offset longitudinally from the load cell(s) of adjacent drive(s). However, a disadvantage of this is that it tends to increase the overall length of the instrument drive assembly.
There is a need for an improved drive assembly for a surgical robot arm.